RF power amplifier output power sensor

ABSTRACT

A system for sensing RF amplifier output power includes an amplifier transistor and two sampling transistors. The two sampling transistors are physically smaller than the amplifier transistor, and are preferably the same size. The first sampling transistor is configured to sample the same RF input signal that is amplified by the amplifier transistor. The second sampling transistor is configured to receive and amplify only a bias network signal. The bias network associated with the transistors includes a selection of components based upon operating parameters as well as actual physical sizes of the transistors. The selection of component values in association with transistor sizes is used to enable generation of a current sensing signal that is proportional to the power level of the RF output signal generated by the amplifier transistor. The RF sensing signal is corrected for bias current effects and bias shifts by subtracting the sampled bias current amplified by the second sampling transistor from the RF current sensing signal.

This is a continuation-in-part of Application Ser. No. 09/384,679, filedAug. 27, 1999.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention generally relates to radio frequency (RF) poweramplifier output power detection techniques and, more particularly, to asystem and method for sensing power consumed by a RF power amplifier.

(2) Description of the Prior Art

Portable wireless transmitter systems generally require that a specifiedradio frequency (RF) output power be delivered to the radiating antenna.Further, many such systems are required to back-off or reduce thetransmitted power to achieve a specific level depending upon signalstrength. To meet the aforesaid requirements, the system architecturegenerally incorporates a closed-loop power control scheme. Typically,this scheme requires a “sampling” of the RF power amplifier output powerthat is subsequently fed back to predetermined control circuitry whichgenerates a control signal that adjusts the output power until it iswithin the specified power level. Such sampling of the output power isdisadvantageous in that it increases the insertion loss between theoutput of the power amplifier and the radiating antenna. Therefore,sampling of the output power decreases the available output power fromthe power amplifier and reduces the overall talk time. Talk time is ameasure of the time a portable transceiver can be in the “talk” modebefore the battery is fully depleted. The power amplifier consumes themajority of the current and therefore dominates in the calculation oftalk time.

A common technique for sampling the output power includes use of adirectional coupler on the output of the power amplifier. The powercoupled from the main signal path is diode detected to generate a videosignal proportional to the amplitude of the RE, voltage delivered to theantenna. Use of directional couplers, however, adds loss to the system,forcing the power amplifier to consume more power thereby reducing thetalk time of the associated radio unit. In typical applications, theaforesaid loss is often 5-10% of the power amplifier output power andrelates to a direct loss in available talk time.

Another common technique for detecting the output power includesmeasurement of the current consumed by the power amplifier. This currentis directly related to the output power generated by the power amplifierand is also fed back to predetermined power control leveling circuitry.This technique is also disadvantageous due to the loss associated withthe current measurement. This current measurement generally requiresthat a series “dropping” element be added between the associated batteryand the power amplifier bias input. The voltage across this element willdetermine the current entering the power amplifier (for a knownresistance across the element). In typical applications, the voltageacross the dropping element will be about 3% of the total batteryvoltage. Because this is a loss in the dc input power to the poweramplifier, the loss of talk time will be even higher than 3% due to theless than 100% dc-rf conversion efficiency of the power amplifier. Forexample, if the power amplifier efficiency is 60%, then the talk timeloss will be 3/0.6 or 5%.

Thus, there remains a need for a new and improved technique for currentsensing associated with RF amplifier power detection.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method relating tocurrent sensing to detect RF amplifier output power. A small currentsensing transistor is added in parallel with a large transistor thatdelivers the high power to the antenna. The current in this sensingtransistor is proportional to the current in the large transistor. Thecurrent in the large transistor is directly related to the output powergenerated by the power amplifier. By using a small current sensingtransistor, talk time loss is reduced by less than 1%. This smalltransistor is dc biased and RF driven proportionally to the large outputtransistor. The small transistor (sampling transistor) is about{fraction (1/250)}^(th) the size of the larger output transistor whichresults in a scaling factor such that the additional current requiredfor sensing is about 0.4% of the total current consumed by the poweramplifier.

In order to compensate for a dominating quiescent bias current at lowpower levels, the present invention incorporates a second small currentsensing transistor that is in parallel with the large transistor and thefirst small current sensing transistor. However, the second smallcurrent sensing transistor does not receive the RF signal driving thelarge transistor and the first small current sensing transistor. Rather,the second small current sensing transistor only receives a biascurrent. Preferably, the second small transistor is the same physicalsize as the first small transistor. This configuration allows the powersensed by the first small current sensing transistor to be corrected forbias current effects and bias current shifts.

Accordingly, one feature of the present invention includes a techniquefor measurement of RF amplifier output power that reduces talk time byless than 1%.

Another feature of the present invention includes a technique formeasurement of RF amplifier output power that is more efficient thanknown measurement techniques.

Still another feature of the present invention includes a technique formeasurement of RF amplifier output power which consumes about an orderof magnitude less power than that consumed by using known techniques.

Yet another feature of the present invention includes a technique formeasurement of RF amplifier output power that compensates for biascurrent effects and bias current shifts.

These and other features of the present invention will become apparentto those skilled in the art after a reading of the following descriptionof the preferred embodiment when considered with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram illustrating a traditionalmethod of RF power sensing known in the art.

FIG. 2 is a simplified schematic diagram illustrating a current sensingsystem constructed according to one embodiment of the present invention.

FIG. 3 is a more detailed schematic diagram illustrating a currentsensing system according to another embodiment of the present invention.

FIG. 4 is a simplified schematic diagram illustrating a current sensingsystem that compensates for bias changes according to another embodimentof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, like reference characters designate likeor corresponding parts throughout the several views. Referring now tothe drawings in general and FIG. 1 in particular, it will be understoodthat the illustrations are for the purpose of describing a preferredembodiment of the invention and are not intended to limit the inventionthereto. In FIG. 1, a simplified schematic diagram illustrates atraditional RF power sensing system known in the art, generallydesignated 100. As stated herein, many RF systems need to know how muchpower is being supplied. The classical solution is to obtain samples ofthe RF energy being supplied via a coupling scheme, rectify the samplesand use the resulting voltage as an indication of the power beingsupplied. A pair of coupled transmission lines 102 are used as thedirectional coupler. The power coupled from the main signal path 104 isdetected via a diode 106, i.e. Schottky diode to generate a videosignal, e.g. PWR_Sense 108, proportional to the amplitude of the RFoutput voltage delivered to the antenna (not shown). As stated above,such directional couplers 102 add loss to the system 100, forcing thepower amplifier 110 to deliver more power, thus reducing the valuabletalk time of the associated radio (not shown).

FIG. 2 illustrates a simplified schematic diagram showing a currentsensing system 200 constructed according to one embodiment of thepresent invention. A radio frequency signal is received by the system200 at the rf-in signal port 202 where it is coupled to a RF amplifiertransistor 204 via coupling capacitor 206. An output matching and biasnetwork 208 provides the necessary dc power and antenna matching toprovide efficient transmission of the amplified RF signal to theassociated antenna 210. The output matching and bias network 208 detailsare not important to the present invention and are not discussed hereinto preserve brevity and enhance clarity associated with the presentdiscussion of the invention. It is sufficient to state that a typicaloutput matching network generally consists of a combination of inductorsand capacitors configured to provide an impedance match between the RFamplifier transistor 204 output impedance and the antenna 210 inputimpedance. FIG. 2 also illustrates an implementation of RF output powersensing that is accomplished in part via a small RF signal sensing(sampling) transistor 212. The small RF sampling transistor 212 isdeployed in parallel with the RF amplifier transistor 204 that isphysically much larger than the small sampling transistor 212. Couplinga small amount of the drive energy from the output stage into a smallparallel device provides an integrated approach for indicating the powersupplied by an amplifier. The small sampling transistor 212 is dc biasedvia a dedicated bias network 214. Details of the dc bias network 214 arenot central to the present invention and so will not be discussed hereinexcept to state that such bias schemes are well known to those skilledin the art and may consist of nothing more than a voltage source coupledto the small sampling transistor 212 via a resistor (not shown). In oneembodiment, the size of the small sampling transistor 212 is about{fraction (1/250)}^(th) the size of the RF amplifier transistor 204. Thepresent invention is not so limited however, and it shall be understoodthat other ratios associated with sizing of the RF amplifier transistor204 and the small sampling transistor 212 will also be effective topractice the present invention. For example, the aforesaid ratio couldjust as well be any number between 1 and 500 or larger. Larger ratioscould also be used in association with dedicated applications andprocessing technologies. With a scaling factor of 250, the additionalcurrent required for sensing is only about 0.4% of the total currentconsumed by the current sensing system 200 RF amplifier transistor 204.This is an order of magnitude less than the loss incurred by powermeasurement and sensing systems known in the art. For example,directional couplers reduce a typical transmitter's total efficiency byabout 5% or more.

FIG. 3 illustrates a more detailed schematic diagram of a currentsensing system 300 according to another embodiment of the presentinvention. With reference to the foregoing discussion above regardingratio sizes, a typical ratio of device sizes for RF amplifier transistor302 and sampling transistor 304 can be 224 to 1. In this case, the RFsignal coupling capacitors 306, 308 will employ a similar ratio whilethe bias resistors 310, 312 will optimally employ a ratio of 1 to 224.The values for coupling the capacitor 306 and bias resistor 310 will beset by the design value associated with the specific applicationparameters. In operation, the RF drive signal through capacitor 306 andcapacitor 308 turns on the RF amplifier transistor 302 and the samplingtransistor 304 proportional to the RF signal amplitude. The currentflowing through the sampling transistor 304 then flows through biasresistor R2 and reduces the voltage across capacitor C2. The outputpower can then be determined by sensing the bias current flowing throughbias resistor R2 at the PWR_Sense junction 314 using techniques familiarto those skilled in the art of voltage/current transformations. Asillustrated in FIG. 2, it can be seen that the varying voltage acrosscapacitor C2 (now hidden inside bias network 214) can also be used in afeedback loop to change the gain of the RF amplifier transistor 204. Adifferential amplifier 218 compares a reference voltage 220 with thechanging voltage across capacitor C2 and changes the DC quiescent biascurrent to affect a change in the RF amplifier transistor 204 gaincharacteristics. The output power can thus be increased or decreased asdesired. Most preferably, the RF signal coupling capacitors 306, 308,bias resistors 310, 312 and transistors 302, 304 are integrated on thesame die, leaving the connections to transistor collectors open tomaximize flexibility in configuring the power sense option desired.

While the embodiments illustrated in FIGS. 2 and 3 provide an improvedtechnique for current sensing associated with RF amplifier powerdetection, there is one disadvantage of the approach. The Q2 a collectorcurrent (304FIG. 3) is composed of the DC quiescent current and theinduced RF current. Therefore, at low power levels, the quiescent biascurrent is the dominating current. If the PWR_Sense voltage across C2 isused to control the RF power level, any change in the bias currentintroduces an error. Such a change in the bias current could result froma change in the temperature or power supply.

FIG. 4 illustrates a simplified schematic diagram showing a currentsensing system 400 that compensates for erroneous contributions of abias current according to another embodiment of the present invention. ARF signal is received by the system 400 at the RF-in signal port 402where it is coupled to a RF amplifier transistor 404 via couplingcapacitor 406. An output matching and bias network 408 provides thenecessary dc power and antenna matching to provide efficienttransmission of the amplified RF signal to the associated antenna (notshown). A small RF sampling transistor 412 is deployed in parallel withthe RF amplifier transistor 404, which is physically much larger thanthe small sampling transistor 412. The small sampling transistor 412 isdc biased via a dedicated bias network 414. As noted above, the size ofthe small sampling transistor 412 may be about 1{fraction (250+L )}^(th)the size of the RF amplifier transistor 404. The present invention isnot so limited, however, and it shall be understood that other ratiosassociated with sizing of the RF amplifier transistor 404 and the smallsampling transistor 412 will also be effective to practice the presentinvention.

In order to compensate for a dominating quiescent bias current at lowpower levels, the system 400 includes a second small sampling transistor416. This second small sampling transistor 416 is also physically muchsmaller than the RF amplifier transistor 404 and is optimally the samephysical size as the first small sampling transistor 412. The secondsmall sampling transistor 416 is deployed in parallel with the firstsmall sampling transistor 412, however, it does not receive any of theRF drive. Rather, the second small sampling transistor 416 receives andamplifies only the bias current from the bias networking 414. As aresult of this configuration, the predicted power can be corrected forbias current effects and bias current shifts. More specifically, asimple differential amplifier 418 connected between the PWR_Sense andPWR_Sense _Reference can be used to cancel erroneous contributions ofthe bias current to the predicted power.

Certain modifications and improvements will occur to those skilled inthe art upon a reading of the foregoing description. By way of example,the embodiments described herein above are based upon specific circuitarchitectures. The present invention is not so limited, however. Itshall be understood that those skilled in the art can use a wide varietyof circuit architectures including electronic and electromechanicalcomponents, either integrated or discrete or combinations thereof topractice the present invention so long as the transistor ratiofunctionality described herein is retained. Also, it shall be understoodthat the present invention is not limited to use of a particularprocessing technology, e.g. HBT, Silicon BJT, CMOS, and the like. Itshould be understood that all such modifications and improvements havebeen deleted herein for the sake of conciseness and readability but areproperly within the scope of the following claims.

We claim:
 1. Amplifier circuitry comprising: a) a radio frequency poweramplifier for amplifying a radio frequency input signal and having: i) aradio frequency amplifier input for receiving the radio frequency inputsignal, ii) a bias input for receiving a bias signal for biasing theradio frequency power amplifier, and iii) a power amplifier outputproviding an amplified radio frequency signal; b) a first transistorcircuit having: i) a first input for receiving the radio frequency inputsignal, ii) a first bias input for receiving the bias signal, and iii) afirst output providing a first output signal having a bias component anda radio frequency component proportionally smaller than the amplifiedradio frequency signal; and c) a second transistor circuit having: i) asecond bias input for receiving the bias signal, and ii) a second outputfor providing second output signal corresponding to the bias component.2. The amplifier circuitry of claim 1 further comprising bias circuitryadapted to provide the bias signal as a function of the first and secondoutputs.
 3. The amplifier circuitry of claim 2 wherein the biascircuitry provides the bias signal to compensate for output power of theamplified radio frequency signal.
 4. The amplifier circuitry of claim 1further comprising power sensing circuitry including a subtractioncircuitry adapted to subtract the second output signal from the firstoutput signal to provide a signal proportional to output power of theamplified radio frequency signal.
 5. The amplifier circuitry of claim 1wherein the first and second transistor circuits consist essentially oftransistors providing substantially similar current gains.
 6. Theamplifier circuitry of claim 5 wherein the transistors of the first andsecond circuits are substantially the same size.
 7. The amplifiercircuitry of claim 5 wherein the current gain of the first and secondtransistor circuits are substantially smaller then a current gain forthe power amplifier circuitry.
 8. The amplifier circuitry of claim 5wherein the power amplifier circuitry is implemented using transistorcircuitry.
 9. The amplifier circuitry of claim 8 wherein the first andsecond transistor circuits are substantially the same size andproportionally smaller than the power amplifier circuitry.
 10. Amplifiercircuitry comprising: a) a radio frequency power amplifier foramplifying a radio frequency input signal and having: i) a radiofrequency amplifier input for receiving the radio frequency inputsignal, ii) a bias input for receiving a bias signal for biasing theradio frequency power amplifier, and iii) a power amplifier outputproviding an amplified radio frequency signal; b) a first transistorcircuit having: i) a first input for receiving the radio frequency inputsignal, ii) a first bias input for receiving the bias signal, and iii) afirst output providing a first output signal having a bias component anda radio frequency component proportionally smaller than the amplifiedradio frequency signal; and c) bias circuitry adapted to provide thebias signal as a function of the first output, wherein the biascircuitry provides the bias signal to compensate for output power of theamplified radio frequency signal.
 11. The amplifier circuitry of claim10 further comprising a second transistor circuit having: a) a secondbias input for receiving the bias signal, and b) a second output forproviding second output signal corresponding to the bias signal, whereinthe bias circuitry is adapted to provide the bias signal as a functionof the second output such that the bias signal compensates for outputpower of the amplified radio frequency signal.
 12. The amplifiercircuitry of claim 10 further comprising power sensing circuitryincluding a subtraction circuitry adapted to subtract the second outputsignal from the first output signal to provide a signal proportional tooutput power of the amplified radio frequency signal.
 13. The amplifiercircuitry of claim 10 wherein the first and second transistor circuitsconsist essentially of transistor s providing substantially similarcurrent gains.
 14. The amplifier circuitry of claim 13 wherein thetransistors of the first and second circuits are substantially the samesize.
 15. The amplifier circuitry of claim 13 wherein the current gainof the first and second transistor circuits are substantially smallerthen a current gain for the power amplifier circuitry.
 16. The amplifiercircuitry of claim 13 wherein the power amplifier circuitry isimplemented using transistor circuitry.
 17. The amplifier circuitry ofclaim 16 wherein the first and second transistor circuits aresubstantially the same size and proportionally smaller than the poweramplifier circuitry.